Method of electrolysis



Patented May 20, 1930 PATENT OFFICE URLYN o. 'rAINToN, or EAST s'r.LoUIs, ILLINOIS METHOD OF ELECTBOLYSIS No Drawing. Application filedApril 1, 1926, Serial No. 99,139. .Renewed May 13, 1929.

This inventi'on relates to improvements in the art of theelectrodeposition of metals and particularly to the depositionof zincfrom sulphate solutions.

In the electrolytic recovery of zinc from ores, the zinc is ordinarilybrought into solution as a sulphate. On account of the fact thatmanganese is universally present in zinc ores, such solutions alwayscontain manganese sulphate in addition to zinc sulphate.

When such a solution is electrolyzed, say

between aluminum cathodes and lead anodes,

as described in my prior Patent No. 1,247 ,17 9,

issued November 20, 1917, zinc is deposited at the cathode and manganesedioxide is precipitated at the anode.

' It is found that under the conditions above described, which are alsothe conditions almost universally employed in practice, the

zinc deposited at the cathode always contains small but importantamounts of lead which definitely reduce the purity of the depositedmetaL- I have found that this impurity is not deposited with the zincfrom solution, but is mechanically introduced in the following way: whenordinary lead anodes are used, some manganese is precipitated as thedioxide in the form of flakes or scales on the anode and thisprecipitated dioxide always contains substantial amounts of lead. Aselectrolysis is continued, small pieces of this scale become detachedand are carried in suspension through the electrolyte and deposited onthe cathode. Lead is thus mechanically occluded in the zinc deposit andlowers the purity of the metal.

I have found that the above effect may be minimized or entirelyprevented in the following Way: instead of using a pure lead for theanode, an alloy of lead with silver and preferably also another metal isused. The

amount of silver required is comparatively smallless than one percent ofthe weight of the lead will give good' results. The amount required,however, is greater than will be found in any ordinary commercial leadmetal. Quantities 1n excess of one percent may, however, often be usedwith advantage.

In this invention, while the lead-silver anode may be used alone, thereare often beneefits to be gained from the addition of small amounts ofother metals as well. Particularly well adapted for this purpose arearsenic and tin.

The use of this anode makes possible a number of improvements in presentday electrolytic zinc practice. Not only is the purity of the zincincreased, but it also becomes possible to recover the manga ese dioxidein a commercially valuable form. Ordinarily the manganese dioxideprecipitate contains considerable quantities of lead which make itvalueless for most commercial purposes; moreover, the deposit is largelyin the form of flakes or scales which are undesirable for I industrialuses. For these reasons the manganese dioxide produced at the zincplants has little if any market value. Under'the present invention, themanganese dioxide is precipitated in the form of a very finely di-'vided powder, and contains so little lead that it may be employeddirectly for drycells, varnish making, ink manufacture and otherindustrial purposes.

Moreover, owing to the absence from the electrolyte of the flakes ofmanganese dioxide above mentioned, it is found that the depositedzinc ismuch smoother than normally, and the growth of nodules or trees on thedeposit is prevented. This is ofgreat importance because it permits theelectrodes to be spaced nearer together without danger ofshortcircuiting, and so reduces power consumption which is ordinarilythe largest item in electrolytic zinc costs.

Yet another advantage is that the extraction-of the zinc from the oremay be materially raised. In the ordinary process ofzinc electrolysis,sulphuric acid is set free at the anode in amount exactly correspondingto the zinc deposited at the cathode and no more. In order to make theacidbalance therefore in the leaching step, it is necessary to usenothing but zinc in the form of oxide to neutralize the acid. Towardsthe end of the neutralization as the acid becomes Weaker, the extractionof zinc oxide from the roasted ore becomes quite ineflicient. It wouldbe of-advantage at this stage to finish .and therefore in the ore.

' source.

It has been proposed to add extra manganese sulphate to the circuit toprecipitate manganese dioxide at the anode and set free. more sulphuricacid. Hitherto, owing to the V fact that the precipitated manganesedioxide had no commercial value, such a course was commerciallyimpracticable as the cost of the manganese sulphate would exceed thebenefit of increased zinc extraction which the extra acid allows. Withthe present invention, however, since the manganese, as dioxide, may besold at a higher price than the manganese, as sulphate, will cost, thereis a clear gain in increased acid regeneration possible zinc recoveryfrom In carrying this invention into effect I prefer the followingprocedure. Anodes are made from lead to which has been added enoughsilver to bring the total silver content up to, say, two hundred andfifty ounces of silver per ton of alloy. Arsenic may also be added inamount, say, of one to five percent according to the hardness of leadrequired. If a hard lead anode is not wanted, tin may be substituted forarsenic.

I prefer to make the anodes in the form of a flat sheet,perforatedfreely with holes. I recommend the use of two such sheets, about oneinch apart, between each pair of cathodes. This permits a lower voltageand a better circulation of the electrolyte than when only one sheet isused. Both cathodes and anodes should be so placed in the cell that thefaces are kept strictly parallel. This may be conveniently done by theuse of grooved guides of wood, hard rubber, or other non-conductingsubstance fixed in the cell and holding the edges of the electrodes.Because of the smooth nature of the zinc and also the fact that theanodes so made stay fiat and do not buckle, the faces of the electrodesmay be as close as five-eights of an inch. When the zinc on the cathodebecomes sufficiently thick the cathode is withdrawn, the zinc strip eand the cathode placed back in the guides The manganese dioxide falls tothe bottom of the cell as a fine powder and may be sluiced out asoccasion requires, filtered, washed, and

sent to market.

On account of the expense of making the whole anode of lead silveralloy, it may be desirable to use the silver only at the work which hasa core of arsenical lead with a layer of lead-silver on each side. Whenthis is rolled into a sheet say one quarter inch in thickness, thelead-silver layer on each side may be only one thirty second inch thickwith a three sixteenths layer of 'arsenical lead between.

Having thus described my invention, what I claim and desire to secureby'Letters Patent is 1. An anode for use in the electrolysis of zinc andmanganese salts,said anode consisting of an alloy containing lead andsilver. 2. An anode for use in the electrolysis of zinc and manganesesalts, said anode consisting mainly of lead but containing a substantialproportion of silver.

3. An anode for use in the electrolysis of zinc and manganese salts,said anode consisting mainly of lead. but containing substantially onepercent of silver. v

4. An anode for use in the electrolysis of zinc and manganese salts,said anode consisting mainly of lead and containing substantialproportions of silver and-arsenlc.

5. An anode for use in the electrolysis of as posed of an alloyconsisting mainly of lead ut containing a substantial proportion ofsilver.

8. In the electrolysis of iinc sulphate solutions an anode having anactive surface composed of an alloy consisting mainly of lead butcontaining silver to'the extent of one part per'thousand or more oflead.

URLYN o. TAINTON.

